1. Field of the Invention
This invention relates generally to output pads and, more specifically, to a programmable output pad having circuitry for reducing ground bounce noise and power supply noise and method therefor.
2. Description of the Related Art
In general, computer manufacturers have been designing and producing smaller personal computers such as the laptop computer. This trend of reducing the physical size of computers is largely the result of the success the industry has had in increasing the number of electronic components per unit area on a semiconductor chip. However, the problem of battery power conservation as it relates to the operation of a laptop computer is that a laptop computer operating under battery power typically has only 2-3 hours of operation before the battery must be recharged. There are several possible solutions to the problem of limited battery powered operation for a laptop computer. For example, a laptop computer user can configure the computer in order to extend the battery powered operational time of the laptop by taking such actions as dimming the computer display and reducing the computer's clock speed. Such energy conservation steps make the usage of the laptop computer less enjoyable and less time efficient. Most personal computer users that have shifted from a desktop to a laptop personal computer are reluctant to sacrifice system performance in order to extend the time under which the laptop can operate on battery power.
In an effort to extend the uninhibited operation of the laptop computer while under battery power and in order to make other personal computers such as the desktop personal computer more energy efficient, computer designers are reducing the logic level power supply from the previous industry standard of 5 volts to a new, lower logic level power supply of 3 volts. Currently, most computer systems operate at either 5 or 3 volts. Consequently, it is desirable to design components that can operate effectively at either 5 volts or 3 volts, or some other pair of voltages.
Output pads that are designed to operate effectively at either 5 or 3 volts are subject to the ground bounce noise phenomenon as well as similar types of noise on the 5 or 3 volt power supply. Those skilled in the art may refer to this noise problem on a power supply such as a 5 or 3 volt power supply as voltage droop. Additionally, the ground bounce and power supply noise problems are well known to those skilled in the art. In general, this problem is described as follows. The conductive metal links used for power and ground feeds to components on the output pad posses an inherent amount of inductance. For a single output pad which is powered by these links, when the output pad is switched from one logic level to another, the current flowing through these links is relatively small. However, in a large integrated circuit design, several outputs may change states simultaneously. The combined magnitude of the current variations of these outputs through their power and ground feeds results in the generation of non-trivial induced voltage. This induced voltage is proportional to the time rate of change of the current flowing through the power and ground feeds times the inductance associated therewith. This induced voltage may be applied through the output pad driver's output and may be mistakenly interpreted by a load device as a change in logic, thereby creating erroneous data.
This ground and power supply noise problem becomes more prevalent when an output driver is designed to operate with a 3 volt power supply. For example, if the drive components of an output driver are designed for operation with a 3 volt power supply, but a 5 volt power supply is used, then the increased drive power associated with the 5 volts power supply operation causes the aforementioned ground and power supply noise problem to be more severe at 5 volts than it would have otherwise been at 3 volts. Additionally, if an output driver is designed for driving a relatively heavy load, but a lighter load is actually used, then more ground and power supply noise will result when the load is lighter than the design load. Until now, there has not been an output driver that can be used with different power supply voltages and/or different load conditions while reducing the detrimental effects of the resultant ground and power supply noise which are prevalent when a higher than design power supply is used and/or when a lighter than design load is used.